1. Technical Field
Embodiments of the present invention generally relate to a magnetic permeability detector, a developer density detector, a developing device, and an image forming apparatus, such as, a copier, a printer, a facsimile machine, a plotter, or a multifunction peripheral (MFP) having at least two of coping, printing, facsimile transmission, plotting, and scanning capabilities; and further relates to a method of calculating a frequency of an oscillation signal.
2. Description of the Background Art
There are sensors that employ an LC oscillator circuit including a coil provided on a plane and detect magnetic permeability of space opposed to the plane on which the coil is provided (hereinafter “coil formation face”) according to the frequency of signals output from the LC oscillator circuit. For example, in JP-H11-223620-A, such a sensor is used to detect the density of developer including magnetic substances inside a container in an electrophotographic image forming apparatus. The principle of such magnetic permeability sensors is based on that the inductance of coil changes depending on the magnetic permeability of a magnetic material to be detected. The inductance of coil changes due to changes in physical properties, such as magnetic permeability and conductivity, of the magnetic substance and changes in the distance to the magnetic material. The magnetic permeability in a range in which the magnetic flux of the magnetic permeability sensor acts can be detected by reading the oscillation frequency of the LC oscillator circuit dependent on inductance changes. In JP-H11-223620-A, further a resistance component of the circuit is considered in addition to respective elements of the circuit for designing with a higher degree of accuracy.